Soft ablative desorption method and system

Abstract

Methods and systems are provided for the soft desorption of analyte from a sample, in which an optical beam absorbed within an irradiate zone of the sample causes vibrational excitations of a component within the sample. The optical beam, providing sufficient energy to superheat the component, is provided for a time interval that is less than the time duration required for the loss of energy out of the irradiated zone due to thermal diffusion and acoustic expansion. The superheated component thus drives ablation within the irradiated zone, resulting in the soft desorption of analyte without ionization and fragmentation. The ejected ablation plume may be directed towards the inlet of a mass analysis device for detection of the desorbed analyte, which is preferably ionized by a linear resonant photo-ionization step.

1. A method of performing the soft desorption of analyte from a sample, said method comprising the steps of:
 irradiating said sample with an optical beam; and 
 optically exciting vibrational levels of a component of said sample within an irradiated zone; 
 wherein said sample is irradiated for a time interval that is less than a time duration required for energy loss beyond said irradiated zone to thermal diffusion and acoustic expansion; and 
 wherein energy absorbed from said optical beam by said component is sufficient to superheat said component and cause the ejection of an ablation plume from said irradiated zone. 
 
     
     
       2. The method according to  claim 1  wherein said ablation plume comprises said analyte in a gas phase. 
     
     
       3. The method according to  claim 1  wherein said analyte desorbed from said sample is desorbed in an unexcited state. 
     
     
       4. The method according to  claim 1  wherein analyte desorbed from said sample is desorbed in a neutral state. 
     
     
       5. The method according to  claim 1  wherein a concentration of said component is such that said irradiated zone has a depth on a micron scale. 
     
     
       6. The method according to  claim 1  wherein said component is water. 
     
     
       7. The method according to  claim 6  wherein said optical beam excites one of OH stretching vibrations, OH bend modes, and a combination thereof. 
     
     
       8. The method according to  claim 6  wherein an energy flux of said optical beam at said sample is at least 0.1 J/cm 2 . 
     
     
       9. The method according to  claim 6  wherein said time interval is between approximately 100 fs and 1 ns. 
     
     
       10. The method according to  claim 9  wherein said time interval is less than 100 ps. 
     
     
       11. The method according to  claim 1  wherein a transverse spatial profile of said optical beam is substantially uniform in intensity. 
     
     
       12. The method according to  claim 1  wherein said optical beam comprises a laser pulse. 
     
     
       13. The method according to  claim 1  wherein an intensity of said optical beam is less than a threshold for multi-photon absorption. 
     
     
       14. The method according to  claim 1  wherein said sample comprises a biological sample. 
     
     
       15. The method according to  claim 14  wherein said sample is selected from the list comprising tissue, cells, organs, bodily fluids, urine, blood, serum, plasma, cerebral spinal fluid, and sputum. 
     
     
       16. The method according to  claim 1  wherein a thickness of said sample is approximately equal to an absorption depth of said optical beam. 
     
     
       17. The method according to  claim 16  wherein said sample is provided on a substrate, wherein said substrate is transparent to said optical beam, said method comprising the step of irradiating said sample through a back surface of said substrate. 
     
     
       18. The method according to  claim 16  wherein said sample is a liquid sample, and said liquid sample is provided in a microwell. 
     
     
       19. The method according to  claim 18  wherein said microwell is formed in a substrate, wherein said substrate is transparent to said optical beam, said method comprising the step of irradiating said sample through a back surface of said substrate. 
     
     
       20. The method according to  claim 19  wherein said substrate comprises a material selected from the list comprising silicon, sapphire and SiO 2 . 
     
     
       21. The method according to  claim 1  further comprising the step of providing an array of samples on a sample holder, and wherein said steps of performing soft desorption of analyte from a sample is repeated for each sample within said array of samples. 
     
     
       22. The method according to  claim 21  wherein said array of samples comprises an array of liquid samples, each said sample contained within a well. 
     
     
       23. The method according to  claim 22 , further comprising the step of filling said wells by a capillary prior to said step of irradiating said sample. 
     
     
       24. The method according to  claim 23  wherein said capillary is a component of a separation system employed to separate analyte within said sample prior to said step of irradiating said sample. 
     
     
       25. The method according to  claim 24  wherein a position of a well filled from said capillary is correlated with a dispensing sequence. 
     
     
       26. The method according to  claim 23  wherein said capillary is employed as a sampling device prior to dispensing said sample into said wells. 
     
     
       27. The method according to  claim 21  wherein said sample holder is housed said housing comprising:
 a support platform for receiving said sample holder; 
 a reservoir for maintaining a vapor pressure within said housing; 
 a slidable cover removably attached to said support platform for enclosing said sample holder and said reservoir within said housing, said cover further comprising an aperture for providing external access for said optical beam to at least one well when said sample holder is enclosed within said housing, and wherein said cover may translated to positioned said aperture over each of said one or more wells while enclosing a remainder of said one or more wells and said reservoir; 
 said method further comprising the steps of: 
 placing said sample holder onto said support platform; 
 filling said reservoir with a liquid selected to stabilize a meniscus within said wells when said wells are filled; 
 enclosing said sample holder and said reservoir with said cover; and 
 translating said cover to position said aperture over said given well when performing said steps of soft desorption of analyte. 
 
     
     
       28. The method according to  claim 1  wherein said optical beam is a second optical beam, and wherein analyte resides within said sample in a complex, said method further comprising the steps of:
 prior to said step of irradiating said sample with said second optical beam, irradiating said sample with a first optical beam; 
 wherein said first optical beam has a frequency selected to cause dissociation of said complex. 
 
     
     
       29. The method according to  claim 28  wherein said complex is a protein complex bound by an amide bond, and wherein a wavelength of said first beam is approximately 1600 cm −1  for the breaking of said amide bond. 
     
     
       30. The method according to  claim 16  wherein said optical beam is a first optical beam, said method further comprising the steps of:
 positioning a substrate comprising a well above said sample prior to irradiating said sample with said first optical beam, said well having a liquid contained therein, wherein said well is positioned to receive at least a portion of said ablation plume within said liquid; 
 receiving said ablation plume within said liquid; 
 irradiating said liquid with a second optical beam; and 
 optically exciting vibrational levels of a component of said liquid within an irradiated zone of said liquid; 
 wherein said liquid is irradiated for a time interval that is less than a time duration required for energy loss beyond said irradiated zone to thermal diffusion and acoustic expansion; and 
 wherein energy absorbed from said second optical beam by said component of said liquid is sufficient to superheat said component of said liquid and cause the ejection of an ablation plume from said irradiated zone of said liquid. 
 
     
     
       31. The method according to  claim 30  wherein said liquid comprises an agent for releasing said analyte from a complex. 
     
     
       32. The method according to  claim 31  wherein said agent comprises a denaturing agent. 
     
     
       33. The method according to  claim 31  wherein said sample comprises a heterogeneous sample, wherein said analyte is initially bound in a complex, and wherein said agent dissociates said complex prior to said step of irradiating said liquid with said second optical beam. 
     
     
       34. The method according to  claim 30 , wherein said substrate comprises an array of wells, each said well having a liquid contained therein, wherein two or more of said wells receive a portion of said ablation plume for providing spatially resolved desorption of said sample, and wherein said steps of irradiating said liquid with a second optical beam and optically exciting vibrational levels of a component of said liquid within an irradiated zone of said liquid are serially performed for each of said two or more wells on a single-well basis. 
     
     
       35. The method according to  claim 34  wherein said liquid comprises an agent for releasing said analyte from a complex. 
     
     
       36. The method according to  claim 35  wherein said agent comprises a denaturing agent. 
     
     
       37. The method according to  claim 35  wherein said sample comprises a heterogeneous sample, wherein said analyte is initially bound in a complex, and wherein said agent dissociates said complex prior to said step of irradiating said liquid with said second optical beam. 
     
     
       38. The method according to  claim 34  wherein said each well in said array of wells comprises a diameter on the micron scale for spatially resolving analyte desorbed from said sample. 
     
     
       39. The method according to  claim 34  wherein said each well in said array of wells comprises a diameter on the sub-micron scale for spatially resolving analyte desorbed from said sample, and wherein said substrate is transparent to said second optical beam, and wherein said second optical beam is delivered by a near-field optical element. 
     
     
       40. The method according to  claim 1  further comprising the step of analyzing said desorbed analyte with an analytic device. 
     
     
       41. The method according to  claim 40  wherein said analytic device is a mass analyzer, and wherein said method further comprises the step of ionizing said desorbed analyte prior to said step of analyzing said desorbed analyte. 
     
     
       42. The method according to  claim 41  wherein said mass analyzer is selected from the list comprising a sector mass spectrometer, a time-of-flight mass spectrometer, an ion-trap mass spectrometer, a quadrupole mass spectrometer, and combinations thereof. 
     
     
       43. The method according to  claim 41  wherein said step of ionizing said desorbed analyte is achieved using an ionization method selected from the list comprising coronal discharge ionization, electrospray ionization, chemical ionization, and thermal emission ionization. 
     
     
       44. The method according to  claim 41 , said method further comprising the steps of positioning said sample so that said ablated plume is directed into the inlet of a mass spectrometer for subsequent mass analysis. 
     
     
       45. The method according to  claim 44  wherein a spacing between said sample and said inlet is approximately 100 microns, and wherein a pressure in the region between said sample and said inlet maintained at a prescribed level by differential pumping. 
     
     
       46. The method according to  claim 45  wherein said pressure is maintained at approximately 5 Torr. 
     
     
       47. The method according to  claim 41  wherein said desorbed analyte is photo-ionized within an evacuated region prior to mass analysis, said method further comprising the steps of:
 positioning said sample so that said ablated plume passes through an aperture into said evacuated region; 
 irradiating said ablation plume within said evacuated region with a vacuum-ultraviolet beam having a wavelength selected to photo-ionize said desorbed analyte; and 
 performing mass analysis of said ionized analyte. 
 
     
     
       48. The method according to  claim 47  wherein a spacing between said sample and said aperture is approximately 100 microns, and wherein a pressure in the region between said sample and said inlet maintained at a prescribed level by differential pumping. 
     
     
       49. The method according to  claim 48  wherein said pressure is maintained at approximately 5 Torr. 
     
     
       50. The method according to  claim 47  wherein said aperture is an inlet to said mass spectrometer, and said evacuated region is within said mass spectrometer. 
     
     
       51. The method according to  claim 47  wherein said sample is optically thick and recoil material is ejected after said ablation plume, said method comprising the step of irradiating only said ablation plume, so that said recoil material is not ionized. 
     
     
       52. The method according to  claim 47  wherein an interaction length between said vacuum ultraviolet beam and said ablation plume is extended using an optical cavity formed within said evacuated region. 
     
     
       53. The method according to  claim 44  wherein said desorbed analyte is photo-ionized prior to mass analysis through a linear resonant process, said method further comprising the steps of:
 irradiating said ablation plume with a photo-ionizing beam, wherein said photo-ionizing beam comprises a frequency spectrum and a phase profile for the resonant linear photo-ionization of said desorbed analyte; and 
 performing mass analysis of said ionized analyte. 
 
     
     
       54. The method according to  claim 53  wherein said ablation plume is photo-ionized by said photo-ionizing beam prior to entering said inlet. 
     
     
       55. The method according to  claim 53  wherein said frequency spectrum comprises spectral components for producing one-photon linear absorption between adjacent levels to ionize said analyte and a phase profile selected to provide the sequential one-photon excitation between said levels from a first level to an ionized state. 
     
     
       56. The method according to  claim 55  wherein said ablation plume comprises a plurality of molecular species, and wherein said analyte is selectively ionized. 
     
     
       57. The method according to  claim 55  wherein a spacing between said sample and said inlet is approximately 100 microns, and wherein a pressure in the region between said sample and said inlet maintained at a prescribed level by differential pumping. 
     
     
       58. The method according to  claim 57  wherein said pressure is maintained at approximately 5 Torr. 
     
     
       59. The method according to  claim 44  wherein said desorbed analyte is photo-ionized prior to mass analysis through a two-photon resonant process, said method further comprising the steps of:
 irradiating said ablation plume with a photo-ionizing beam, wherein said photo-ionizing beam comprise a frequency spectrum for exciting said analyte to a first excited state by two photon absorption, driving said excited state into saturation, and ionizing said excited analyte in a linear regime; and 
 performing mass analysis of said ionized analyte. 
 
     
     
       60. A method of preparing a sample for mass analysis, said method comprising the steps of:
 irradiating a sample with an optical beam; and 
 optically exciting vibrational levels of a component of said sample within an irradiated zone; 
 wherein said sample is irradiated for a time interval that is less than a time duration required for energy loss beyond said irradiated zone to thermal diffusion and acoustic expansion; 
 wherein energy absorbed from said optical beam by said component is sufficient to superheat said component and cause the ejection of an ablation plume from said irradiated zone; 
 ionizing said ablation plume with an ionizing means; and 
 directing said ionized ablation plume into an inlet of a mass analysis device. 
 
     
     
       61. A method of performing mass analysis on a sample, said method comprising the steps of:
 irradiating said sample with an optical beam; and 
 optically exciting vibrational levels of a component of said sample within an irradiated zone; 
 wherein said sample is irradiated for a time interval that is less than a time duration required for energy loss beyond said irradiated zone to thermal diffusion and acoustic expansion; 
 wherein energy absorbed from said optical beam by said component is sufficient to superheat said component and cause the ejection of an ablation plume from said irradiated zone; 
 ionizing said ablation plume with an ionizing means; 
 directing said ionized ablation plume into an inlet of a mass analysis device; and 
 performing mass analysis on said ionized ablation plume. 
 
     
     
       62. A system for desorbing an analyte, wherein said sample comprises a component having an excitation spectrum comprising vibrational energy levels, said system comprising:
 an optical apparatus for directing an optical beam onto a sample and irradiating an irradiation volume within said sample, said optical beam comprising: 
 a frequency spectrum for optically exciting vibrational levels of a component of said sample; 
 a time duration shorter that is less than a timescale required for energy loss beyond said irradiated zone to thermal diffusion and acoustic expansion; and 
 sufficient energy to superheat said component and cause ejection of an ablation plume from said irradiated zone. 
 
     
     
       63. The system according to  claim 62  wherein said component is water and wherein an energy flux of said optical beam at said sample is at least 0.1 J/cm 2 . 
     
     
       64. The system according to  claim 62  wherein said time duration is between approximately 100 fs and 1 ns. 
     
     
       65. The system according to  claim 64  wherein said time interval is less than 100 ps. 
     
     
       66. The system according to  claim 62  wherein a transverse spatial profile of said optical beam is substantially uniform in intensity. 
     
     
       67. The system according to  claim 62  wherein said optical source apparatus comprises a laser and said optical beam comprises a laser pulse. 
     
     
       68. The system according to  claim 62  wherein an intensity of said optical beam is less than a threshold for multi-photon absorption. 
     
     
       69. The system according to  claim 62  wherein an absorption depth of said optical beam is approximately equal to a thickness of said sample. 
     
     
       70. The system according to  claim 62  further comprising a substrate for supporting said sample, wherein said substrate is transparent to said optical beam, wherein said beam is directed through a back surface of said substrate onto said sample. 
     
     
       71. The system according to  claim 62  wherein said sample is provided in a microwell formed in a substrate, wherein said substrate is transparent to said optical beam, wherein said beam is directed through a back surface of said substrate. 
     
     
       72. The system according to  claim 62  further comprising an array of microwells supported on a sample holder and a capillary apparatus for filling said microwells with a plurality of samples. 
     
     
       73. The system according to  claim 62  further comprising:
 an ionization means for ionizing said ablation plume; and 
 a mass spectrometer for performing mass analysis on said desorbed analyte; 
 wherein said sample is positioned to direct said ablation plume through an inlet of said mass spectrometer. 
 
     
     
       74. The system according to  claim 73  wherein ionization means is selected from the list comprising a coronal discharge ionization apparatus, electrospray ionization apparatus, chemical ionization apparatus, and thermal emission ionization apparatus. 
     
     
       75. The system according to  claim 73  wherein a spacing between said sample and said inlet is approximately 100 microns, and wherein a pressure in the region between said sample and said inlet maintained at a prescribed level by differential pumping. 
     
     
       76. The system according to  claim 73  wherein said ionization means comprises a second optical apparatus for directing a vacuum-ultraviolet beam onto said ablation plume within an evacuated region, wherein said vacuum-ultraviolet beam photo-ionizes said desorbed analyte. 
     
     
       77. The system according to  claim 76  wherein said evacuated region comprises an optical cavity for increasing an interaction length between said vacuum ultraviolet beam and said ablation plume. 
     
     
       78. The system according to  claim 73  wherein said ionization comprises a second optical apparatus for directing a photo-ionizing beam onto said ablation plume, wherein said photo-ionizing beam comprises a frequency spectrum and a phase profile for the resonant linear photo-ionization of said desorbed analyte. 
     
     
       79. The system according to  claim 78  wherein said photo-ionizing beam is directed onto said ablation plume on an external side of said inlet. 
     
     
       80. The system according to  claim 78  wherein said frequency spectrum comprises spectral components for producing one-photon linear absorption between adjacent levels to ionize said analyte and a phase profile selected to provide the sequential one-photon excitation between said levels from a first level to an ionized state.